Spend $75, get the SheppardAir software, study for a few days, and take the 121 version. It makes absolutely no difference whether you take the 121 or 135. When you get the rating, an ATP is an ATP.
Plus, SheppardAir guarantees you score a 90 or better, or you get your money back. If you see a question on the test that they didn't show you in their software, you get your money back. Very easy to use and very helpful. It took me about two days of intense studying to feel comfortable enough to take the test, and I scored a 93.
Good luck with the test!